Table 5.9 and Table 5.10 give ACS model PSFs in the central 5 × 5 pixel region in two filters. Numbers listed are the fraction of the total energy received in each pixel. The models have been generated using Tiny TIM, taking into account the HST optical aberrations and obscurations as well as the CCD pixel response function. Field dependent geometrical distortions are included. The real PSF will also differ from the model because of the jitter in the HST pointing, HST focus variation (focus breathing), and other instrumental effects, some of which are briefly discussed below. The spatial PSF variations of the WFC and HRC detectors are fully described in ACS ISR 2004-15and ACS ISR 2006-01.

Geometric distortions produce a significant impact on the shape of the PSF in all three of the ACS channels, as can readily be seen in Figure 5.15 and Figure 5.16, which display WFC and HRC PSF images. The logarithm stretch enhances the spider diffraction patterns, which the distortion renders non-perpendicular, and the outer Airy rings, which appear elliptical. The distortion owes primarily to the tilt of the focal surface to the chief ray at the large OTA field angles of the ACS apertures. The linear, field-independent approximation for the WFC produces a difference in plate scale of about 8% between the two diagonals of the field and, in the HRC and SBC, about a 16.5% difference in scale between orthogonal directions rotated about 20° from the aperture edges. Field-dependent distortions, measured as actual vs. predicted distances from field center, amount to about 2% peak in the WFC and about 1% in the HRC and SBC. The distortion solutions are stable following the WFC repair.

At the extreme corners of the WFC field, increased astigmatism slightly elongates the PSF core. The axis of elongation rotates by 90° if the system passes through focus due to breathing. This may affect ellipticity measurements of small galaxies with bright cores at the field edges. Focus variations in the WFC, which alter the amount of light in the peak, are largely due to detector surface height irregularities and amount to the equivalent of 5 microns of breathing (1/18 wave rms). The largest focus offset is along the gap between the two CCDs. Variations in the width of the PSF core are dominated by changes in CCD charge diffusion, which is dependent on the thickness of the detector (12 to 17 microns for the WFC). The PSF FWHM in F550M, for example, can vary by 20% (0.10 to 0.13 arcseconds) over the field. More information about the dependence of the WFC PSF on OTA temperature and breathing is provided in ACS ISR 2007-12 and ACS ISR 2008-03.